(a) Field
Apparatuses employed for the containment of plasmas by magnetic fields utilize many varied configurations. Two well-known categories of these machines are the open-ended type such as the magnetic mirror and the toroidal type such as the tokamak and the stellarator. One advantage to the toroidal type is that a trapped charged particle must move laterally across magnetic field lines to escape confinement. As a result both ions and electrons are confined in a toroidal apparatus for many collision times. (Unless stated otherwise ion shall be taken to mean positive ion hereinafter.) Since the ions tend to remain in a spiral orbit about a givem set of magnetic field lines, the continuity of the magnetic field lines inside the apparatus tends to ensure containment.
Apparatuses of the open-ended type have the disadvantage that the trapped charged particles may escape while traveling along the magnetic field lines which define their spiral orbits. The magnetic field lines do not close on themselves inside the simple magnetic mirror. As a result, the simple magnetic mirror suffers large plasma losses out the mirror ends. The net positive potential of the confined plasma adds to the losses since the ions are confined better than electrons in a simple magnetic mirror. One early mirror confinement apparatus is disclosed in Post, U.S. Pat. No. 3,170,841, filed on July 14, 1954.
The physics of a simple magnetic mirror is discussed at length in the Post patent as well as in Samual Glasstone and Ralph H. Loveberg, Chapter 9, "Magnetic Mirror Systems," Controlled Thermonuclear Reactions, D. Van Nostrand Co., Inc., Princeton, New Jersey (1960), p. 336, et seq and in David J. Rose and Melville Clark, Chapter 10, "Motion of the Individual Charges," Plasmas and Controlled Fusion, John Wiley & Son, Inc., New York (1961) p. 198, et seq.
(b) Prior Art
The problem of end losses in magnetic mirrors has been addressed in a number of ways. One approach links several mirrors together to form roughly a toroidal configuration with magnetic field lines closed inside the apparatus. Particles which leak out of one magnetic mirror simply leak into an adjacent magnetic mirror. Post noted this in FIG. 25 of U.S. Pat. No. 3,170,841, supra. Other closed systems of linked magnetic mirrors include Dandl, U.S. Pat. No. 3,728,217. Each magnetic mirror segment is independent of the next, the total effect on the toroidally confined plasma being a stabilization and confinement of the plasma, which is by means other than by electrostatic plugs.
In linked three-cell systems the earliest prior art appears in FIG. 22 of Post, U.S. Pat. No. 3,170,841. However Post's three-cell system does not operate as three cells simultaneously. The end cells exist as thermonuclear reaction zones alternately and do not serve to electrostatically stopper the central cell.
A three-mirror system to change the potential at the linking magnetic mirrors is suggested by G. G. Kelley, 9 Plasma Physics 503 (1967). Since electrons travel more freely through the mirroring regions than ions, the mirroring regions have a net negative charge. Thus, ions which would have mirrored are drawn deeper into the mirroring region, and some are lost. To overcome this enhanced end loss Kelley injected cold neutral species into the mirroring regions of the center mirror cell of a three mirror cell system. The cold neutral species ionize; thus, these mirroring regions substantially lose their negative potential. Kelley did not try to make the end mirror cells electrostatic end plugs to stop end-losses in the center mirror cell. He addressed a problem of enhanced end losses without touching on the basic end-loss problem in an open-ended system. For three similar mirrors heating may be desired but not cooling. Kelley never considered his cold neutral beam sources for cooling.
The tandem mirror is a method and apparatus for confining a plasma in a center mirror cell by use of two end mirror cells as positively charged end stoppers. Leakage of ions from the center mirror cell is minimized. T. K. Fowler first published this idea in T. K. Fowler, End Stoppering in Mirror Machines, Lawrence Livermore Laboratory Rept. UCID-17244 (1976). G. I. Dimov, V. V. Zakaidakov, and M. E. Kishinevsky, 2 Soviet Journal of Plasma Physics 597 (July-August 1976) is a publication of the tandem mirror idea as well. In neither publication is the cooling of the ions in the center mirror cell considered.
Once the temperature for the ions in the center mirror cell is selected, the tandem mirror can confine these ions through the selection of the depth of the potential well between the end mirror cells. However heating can take place, and a hotter ion distribution includes more ions with the energy to climb out of the potential well. Above the temperature of ions desired to be confined in the center mirror cell any temperature increase means less effective end plugging. Simply choosing very dense end mirror cell plasma is expensive in power and capital costs. Thus a way must be found to keep the center mirror cell plasma temperature down. If the entire tandem mirror is made large enough, the problem goes away in scaling, but building a very large machine is expensive. Neither Fowler nor Dimov, et al suggested a means of cooling for the center mirror cell plasma.